1. Field of the Invention
This invention is concerned with ornamental articles having high resistivity to scratching, light, attack by various chemicals, etc. More particularly, it relates to natural or cultured pearls having such high resistivity which are suitable for use in necklaces, chokers, finger rings, brooches, ear rings, necktie pins, cuff buttons and the like.
2. Description of the Prior Art
Few of naturally occurring products can directly be used as ornamental articles. Such products are often subjected to various processes, such as cutting, grinding and boring, depending on their applicable use, their forms and other conditions when produced or found in nature, etc.
Pearls are used as ornamental articles. Purely naturally occurring pearls yield poorly. Most pearls are produced by seeding nuclei to host shellfish, such as pearl oysters, and growing square cylindrical layers, pearl layers or the like in a concentrical configuration. Improved methods of producing such cultured pearls have been proposed: for example, Japanese Patent Application Laid-open No. 59-183638.
An improvement of the quality of cultured pearls has already been attempted and put to practice by coating pearls with an acrylic thermoplastic resin. However, there has not yet been known a technique of making hard coatings on the surface of pearls while maintaining their color tone and luster which are characteristic to naturally occurring products. Thus, ornamental materials such as pearls still involve an inevitable drawback in that they are readily scratched.
On the other hand, particularly with respect to pearls, the survival rate of host shellfish has improved and pearls of good quality may now be obtained, by the recent progress techniques of pearl production by cultivation. However, there has still been problem of damage caused by the contamination of sea water.
If host shellfish are taken up from sea water after a short period immersion so as to improve the survival rate of the shellfish, the pearls thus obtained are poor in luster due to the thin pearl layer and do not have the characteristic pearl color. Conventionally, such pearls are dyed or coated with a thermoplastic resin in order to improve the poor quality. However, such dye or coating membranes exhibit poor resistance to acids, are discolored during use, or are easily scratched or even removed upon collision with metals or the like due to their poor hardness.
Such problems as described above have also been found in other ornamental materials, such as corals.